The present invention relates to an apparatus for controlling the amount of secondary air fed into an intake passage or into an exhaust passage of an internal combustion engine for controlling an equivalent air-fuel ratio (if an air-fuel passage from the intake passage through exhaust passage located upstream of an air-fuel ratio sensor is defined as a working fluid passage, the equivalent air-fuel ratio is defined as a ratio of the amount of air fed into the working fluid passage to the amount of fuel fed into the working fluid passage) within a predetermined range.
In the field of this art, a method is known in which the equivalent air-fuel ratio is detected by an air-fuel ratio sensor, for example, an oxygen concentration sensor for detecting the concentration of oxygen in the exhaust gas, and; then, secondary air is fed into an intake passage or into an exhaust passage of an internal combustion engine according to the detected equivalent air-fuel ratio, for maintaining the equivalent air-fuel ratio within a predetermined range which is near the stoichiometric air-fuel ratio, whereby the effect of purifying pollutants in a three-way catalytic converter disposed in the exhaust system is improved.
In a conventional apparatus for carrying out the above-mentioned method, the amount of secondary air to be injected into the engine is controlled by an air flow control valve disposed in a passage between an air pump and a secondary air injection mechanism. The air flow control valve is driven by an actuating pressure applied thereto through an electromagnetic valve which is adapted for switching the transmission of the actuation pressure on or off in response to an electrical signal provided from the air-fuel ratio sensor. More specifically, when a lean signal, which indicates that the equivalent air-fuel ratio is on the lean side of the stoichiometric air-fuel ratio, is provided from the air-fuel ratio sensor, a diaphragm of the air flow control valve is not actuated by the actuating pressure and is pressed by a return spring, so as to form a passage for discharging the air fed from the air pump into the atmosphere. Further, when a rich signal, which indicates that the equivalent air-fuel ratio is on the rich side of the stoichiometric air-fuel ratio, is provided from the air-fuel ratio sensor, the diaphragm of the air flow control valve is actuated by the actuating pressure against the pressing force of the return spring, so as to form a passage for providing the air fed from the air pump to the secondary air injection mechanism. The actuating pressure may be a negative pressure, such as vacuum pressure provided from an intake manifold of the engine, or a positive pressure, such as discharge pressure of the air pump.
However, in the conventional apparatus of the above described type, the electromagnetic valve is simultaneously driven with the electrical signal provided from the air-fuel ratio sensor and, thus, the air flow control valve is controlled regardless of the flow amount of the exhaust gas. Therefore, when a rapid increase of the exhaust gas flow occurs, in other words, when the engine is rapidly accelerated, the operation of the air flow control valve cannot follow such changes of the engine conditions. As a result, it is very difficult to control the equivalent air-fuel ratio within the predetermined range in such a case.